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Author Topic: Can I change gears in differential to slow cruise RPM  (Read 3453 times)

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Offline Henrius

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Can I change gears in differential to slow cruise RPM
« on: February 18, 2014, 02:25:18 PM »
The 2.0L in my 1973 Runabout has been worked up to produce well over 105hp. I would like to slow down RPM on the highway. A T-5 would be a great solution, but I don't want to undertake the expense and work to swap a tranny.


How practical would it be to change the gears in the differential? Yes, I know my launch speeds in 1st gear and reverse would increase!


Kim Henry
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1973 Pinto Runabout with 2.0 Liter & 4 speed. My first car, now being restored!

Offline amc49

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Re: Can I change gears in differential to slow cruise RPM
« Reply #1 on: February 18, 2014, 07:59:20 PM »
Not as practical as you think. Changing gears in a rear end requires specialized skills and measuring equipment that are getting harder to find nowadays and anyone doing it will know that enough to charge properly for it. Finding the gearset alone will be an eyeopener when you get a proper price. Many mechanics that attempt it are totally incompetent at it and it shows in the final work which often unwinds to be junk parts in a few days or so.

I worked at a parts store for several years and pretty dang funny all the crazy ideas some of the guys who work on cars for a living have about it. Most have not a clue how to change gears correctly. You need someone who does that stuff for a living.




Offline Wittsend

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Re: Can I change gears in differential to slow cruise RPM
« Reply #2 on: February 18, 2014, 08:05:37 PM »
In all probability you have the 6-3/4" rear end.  Unfortunately you would have to remove the ring/pinion and have the new gears properly set up.  That will not come cheap. 

  It would likely be far cheaper just to swap the whole rear end.  Here in So. Cal. the Pick Your Part price is $100 Drum to Drum (meaning complete). They have 50% off sales monthly.  But before that seems like $50 there is a $15 core, and tax and environmental fees that probably push it to $80.  If you get an 8" then you can just swap the center section with easy.  I think I paid $45 out the door for my 8" 3.40 center section.

  BTW, just changing tire diameter has the same effect as changing the rear gears.  The Pinto rear wheel well sure has room for a bigger tire.  There are calculator all over the internet that can give engine RPM for a tire and gear size.  You can have a 185-70-13" tires with 3.00 gears and it might be equal to a 3.40 with 225-60-15" tires.  This is not actual, but I'm throwing it out there for understanding.  Again, use the calculators.

Lastly what ratios are you looking at?
The 8" came with 2.40, 2.75, 2.79 3.00, 3.40 and 3.55.
The 6-3/4" was 2.73, 3.18, 3.27, 3.40 and 3.55.

These numbers were from Mustang II.ORG for 1974-1978 cars.  It is just my opinion, but the best "average" ratio would be the 3.18/3.27 for the 6-3/4" rear if that works for you.  Lastly if you are getting 105 HP out of a 2.0 I'd assume it is in the upper RPM range.  For normal driving you may have lost the lower RPM torque to use numerically lower gears.  As the saying goes you don't get something for nothing.  But there are minor variations to maximize what you have.  I ran (for a short period of time) 225-60-16" Turbo Coupe tires in my 2.0 with 3.40 gears. Can you say BOG? 

Offline amc49

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Re: Can I change gears in differential to slow cruise RPM
« Reply #3 on: February 19, 2014, 02:36:14 AM »
Good point about the tires, I was being stupid not to think of it. Certainly much easier than changing the entire rear end, which is much easier than changing the gears.

Offline Henrius

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Re: Can I change gears in differential to slow cruise RPM
« Reply #4 on: February 20, 2014, 05:12:42 PM »
Point well taken about wheel size, and a friend got 4 bolt Mustang rims to go on his Pinto to increase tire diameter.

The problem is, I am trying to keep it looking original from the outside, and that means stock hubcaps.

Did not know fooling with differential rings was such a hassle. Sounds like a T-5 would be even a little easier.

No, I have good low and high end torque on my engine. I oversized the valves, increased compression to 9.5 to one, put a header on it with oversized exhaust, and added MSD ignition. The cam is only a little hotter than stock so I still get good torque at most RPM ranges. Made the mistake of overcamming it, and realized how important low RPM torque was.

Just would love to see that baby turning over only 2000 RPM when I am cruising down the expressway!
1973 Pinto Runabout with 2.0 Liter & 4 speed. My first car, now being restored!

Offline Wittsend

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Re: Can I change gears in differential to slow cruise RPM
« Reply #5 on: February 20, 2014, 05:58:11 PM »
Well..., I'm waiting on the wife to get home (what's new about that!) so I threw some numbers in the calculator.  I used a 185-70-13" tire as my basis (given you want the stock look).  I used 65 MPH as the speed (given you were looking to reduce highway RPM).

Here is what I get:

Ratio              4-speed (1 to1)        T-5 (.7)

3.55               3343                       2340
3.40               3202                       2241
3.27               3080                       2156
3.18               2995                       2096
3.00               2826                       1978
2.73               2571                       1800
2.40               2260                       1582

(Note that I opted to exclude the 2.75 and 2.79 being they are close to 2.73)

If you will notice you can't get to your desired 2,000 RPM even with 2.40 gears and a 4-speed.  And frankly I don't recommend that low anyway.  A T-5 with 3.40 or 3.55 would drop you right about 1,000 RPM from stock.  It would take 2.40's to get you there with the 4-speed.  You would likely be frying the clutch to get the car rolling with that ratio.

That 2,241- 2,340 range of the T-5  3.40/3.55 would be better suited to a 2.0.  And again, as I said above, if you are getting 105 HP out of it you probably have lost some low end torque.  There again, the 3.97 first gear of the T-5 might help you out.

Lastly, since I spoke of using (or at least trying) larger tires..., I ran Turbo Coupe 225-60-16" (you will need a small spacer to do so) on a 4 speed at 65 MPH and got the following:

Ratio     4-Speed
3.40  =  2723
3.55  =  2916

That would be "around" the 3.00/3.18 ratios with 185-70-13" tires.  And about a 500 to 400 RPM drop respectively.  If you can borrow some 16" Turbo Coupe wheels/tires I think you will find they will noticeable bog your car down from a standing start.  It will be your call if they decrease the RPM enough at 65 MPH.   If you can accomplish that, I think you will understand the rational for the T-5 and 3.40-3.55 gears.  Sorry to say with gear ratios it is a give and take prospect.  The other option is to add more gears and spread out the application of the torque curve.

Tom



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Re: Can I change gears in differential to slow cruise RPM
« Reply #6 on: February 20, 2014, 07:44:28 PM »
When I had a near stock 2.0L in my 72 I decided to "upgrade to a 8"  rear end assy. Unfortunately, it came with 3.00 gears and an open diff. I was slow off the line with 205/60/13  tires and near impossible to autocross ,and the open diff made it worse. Ok for Highway Rpm's though.  Jumped on getting a 3.40 trac lok center section, and went to Slicks that were only 20" diameter. Now it was usable at all RPM's . But then I went to the near race motor (160 HP) and even with 92 octane it was street lethal.  22 inch diameter street tires kept the Rpm's under 4500 at 60 mph But it would jump to 90 @ 6500 with no sweat when the Webers did their thing. And on the good race gas it was even better... God I miss driving this thing on the street! :-\

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Offline amc49

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Re: Can I change gears in differential to slow cruise RPM
« Reply #7 on: February 20, 2014, 08:17:52 PM »
You really don't want to lug a 2 liter at 2000 rpm on the highway, the oil pressure will be lower and could easily wear out bearings way early doing that. You will be off the torque peak as well. I myself can't see going below 2500.......... .......

You pretty much never run a numerically low rear gear with an OD say 5th gear either, the 5th takes place of the long rear gear and allows you to get the overall gearing numbers back up to where car is very driveable in all the other gears except final. You run the highway gear only with like a one to one final drive.

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Re: Can I change gears in differential to slow cruise RPM
« Reply #8 on: February 21, 2014, 03:28:49 AM »


"...Lastly what ratios are you looking at?

The 6-3/4" was 2.73, 3.18, 3.27, 3.40 and 3.55..."




I think that the 6 3/4" also was available with a 3:73.  (early cars with the 1.6l motor)
the only substitute for cubic inches is BOOST!!!

Offline Wittsend

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Re: Can I change gears in differential to slow cruise RPM
« Reply #9 on: February 21, 2014, 12:23:47 PM »
Update: I ran the numbers using the original Turbo Coupe's 225-60-16" tires, and it's 3.55 ratio and T-5.  I got 2041 RPM @ 65 MPH.  While we are talking a 2.3 Turbo motor it is 200 RPM less than my Pinto is now running (same engine 2.3T/trans T-5). And what?? about 500 pounds lighter too?   I only mention this because 2,000 RPM seems doable but not with a N/A 2.0.

BTW, I originally ran a 3.00 with the T/C drivetrain in my Pinto.  For pure acceleration I noticed little difference between it and the 3.40's I'm running now.  HOWEVER, for driving around town the 3.40 made a NOTICEABLE improvement. The 3.00's were just the wrong RPM for a given street speed (25, 35, 45 etc.).  One gear was always too low and the other too high.

So, we have been talking about dropping highway RPM and the potential for bog on the low end, but there is the overall "driving speed" factor as well.

Offline amc49

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Re: Can I change gears in differential to slow cruise RPM
« Reply #10 on: February 24, 2014, 11:11:39 AM »
Absolutely.

Just as a point of reference, I have three 130 hp. zetec cars, a Contour and two Focus. They run OD ATX and once the engines lose some ring seal to drop maybe 20 hp. from age, they clearly show it in the same issue, the PCM has trouble deciding what gear to use going up say a slight incline, it shuttle shifts back and forth too much. The OD is then a bit too much for the 2 liter engine. It works very much more transparently when the engines are new and fresh.